This issue has been brought up by somebody else:
https://forum.videolan.org/viewtopic.php?f=12&t=113916
https://forum.videolan.org/viewtopic.php?f=12&t=114062
https://forum.videolan.org/viewtopic.php?f=14&t=113302
There is no option in preferences>interface to choose a desired interface language.
There is a solution, according to a developer:
Unfortunately, VLC player remains to be in English even if the language is set to Simplified Chinese in the global settings of OS X. This simply does NOT work.You can set your prefered language in the global settings of your Mac. VLC will take the right one.
I tried setting the global language to Swedish. VLC switched to Swedish accordingly. Then I changed the global language to Chinese. This time VLC switched to English, not Chinese. But the "Check for Update" dialog box was still in Swedish.
Simply put, most of the interface language of VLC is English, a small part is Swedish, while the system language is Chinese.
There was an language option in older versions. I just don't understand why it's gone in version 2.1. Any ideas?